In U.S. Pat. Nos. 4,165,473, 4,400,648, 4,737,679, and 4,823,044, the conventional dispenser cathode structures used in electron guns are explained in detail. There are two types of dispenser cathodes for electron guns, an impregnated cathode and a cavity reservoir type cathode. U.S. Pat. Nos. 4,165,473, 4,400,648, and 4,737,679 relate to the impregnated cathode, and U.S. Pat. No. 4,823,044 relates to the cavity reservoir type cathode.
The structures of impregnated cathodes are shown in FIGS. 1 and 2. In the impregnated cathode, as illustrated in FIG. 1, thermoelectron emissive material is impregnated in a porous base 1 which is made of a heat resistance material, such as tungsten. The porous base is a thermoelectron emissive source and is contained within a reservoir 2 in the form of a cup. This reservoir 2 is disposed within the upper portion of a sleeve 3, which also receives a heater 6. Sleeve 3, is supported by a holder 4 connected to the lower portion thereof, and is enclosed by a large-caliber heat shielding tube 5.
The construction of another similar impregnated dispenser cathode is illustrated in FIG. 2. This impregnated dispenser cathode comprises a reservoir 2 containing a porous base 1, a sleeve 3 for supporting and securing the reservoir 2 and for receiving a heating element 6, a suspending ribbon 8 whose lower portion is welded to the lower end of the sleeve 3 and whose upper portion is welded to the upper end of a large-diameter holder 4, and a heat shielding tube 5 which surrounds the sleeve 3 and which is welded to the holder 4.
On the other hand, a cavity reservoir type cathode has a thermoelectron emissive source different from the aforesaid porous base which is contained in the cup-shaped reservoir. The thermoelectron emissive source of the cavity reservoir type cathode comprises thermoelectron emissive material such as tungsten, barium calcium aluminate, etc. and is contained in a reservoir disposed within the upper portion of the sleeve.
The dispenser cathodes having the above-mentioned constructions have much higher current density than that of an ordinary oxide cathode ray tube, and are adapted to be used in an electron gun of a large-scale cathode ray tube or a projecting tube, for example. However, in the electron gun having a conventional dispenser cathode, the voltage characteristics during initial operation are poor and the radiating state of the electron beam is unstable. These problems are caused because an thermoelectron emissive source of the conventional dispenser cathode, i.e. a porous base, is positioned adjacent and in front of a first electrode of an electron gun. During initial operation, the electron beam more rapidly approaches the first electrode. This rapid approach of the electron beam to the first electrode is a result of structural defects in the cathode.
More specifically, as shown in FIGS. 1 AND 2, the sleeve 3 supported by a holder 4 and receiving a heater 6 thermally expands toward the first electrode. If the sleeve expands and the cathode approaches the first electrode, the cut-off voltage used to control the electron beam varies abnormally. As a result, the white balance of the image fails.
In all electron guns, it is inevitable that some parts of the cathode will shift by thermal expansion. In the conventional cathode ray tube, to obviate this problem, the thermal deformation of the cathode is taken into account during generation of the cathode ray tube with various picture quality controls.